1. Field of the Invention
This invention relates to the dust-proof seal structure of an electric motor which is adapted to rotate a disk-shaped rotor, and which is applicable to a motor for driving a disk-shaped rotor such as a hard disk and a polygon mirror.
2. Related Art
FIG. 4 shows a dust-proof seal structure of a conventional motor disclosed by Examined Japanese Patent Application NO Sho 4-22297. In the motor, a frame 12 or a washer 18, being coaxial with the lower ball bearing 40, covers part of the lower seal 41 of the lower ball bearing 40, thus forming a labyrinth structure. The air flow effect of the labyrinth structure substantially blocks the flow of dust such as oil mist from the inside of the bearing.
FIG. 5 shows the structure of another example of the conventional disk driving motor. In the motor, a pair of ball bearings 40 and 50 are mounted on a shaft 10 which is secured to a frame 20, to rotatably support a hub 30. The hub 30 has a cylindrical portion 30a which holds the bearing 40. The frame 20 has a cylindrical portion 30a which holds a stator core 70. The outer cylindrical surface of the cylindrical portion 30a and the inner cylindrical surface of the cylindrical portion 30a are confronted with each other through a small gap, thus forming a seal structure.
However, the conventional disk driving motor constructed as shown in FIG. 4 suffers from the following problems: That is, in the motor, it is impossible to completely block the flow of dust which comes from the inside of the ball bearing through the small gap between the lower seal 41 of the lower ball bearing 40 and the frame 12 or the washer 18. This is because air is present in the small gap between the lower seal 41 and the frame 12 or the washer 18, and the dust, being fine, is gasified, thus leaking out together with the air. The labyrinth structure is provided directly for the ball bearing 40. Hence, when the dust flows out of the ball bearing 40, immediately it flows into the external space where a rotor such as a disk is present, and sticks onto the disk, signal reading means, etc., thus adversely affecting them.
The disk driving motor shown in FIG. 5 also involves problems to be solved. That is, in order to allow the motor to have a high dust-proof effect, the gap between the outer cylindrical surface of the cylindrical portion 30a and the inner cylindrical surface of the cylindrical portion 30a must be made extremely small. Hence, those inner and outer cylindrical surfaces must be machined with high accuracy, which is one of the factors which increases the manufacturing cost of the motor. In addition, in the motor, the inside diameter of the cylindrical portion 30a of the hub 30 on the movable side is different from that of the cylindrical portion 20a of the frame 20 on the stationary side. Hence, not only the outside diameter of the motor is unavoidably large, but also the rotation of the hub 30 is adversely affected in balance. Therefore, the ball bearings 40 and 50 produce noises, and the motor is lowered in oscillation characteristic.